In coagulation, what is the purpose of rapid mix?

• A. To allow micro-flocs to grow larger over time.
• B. To quickly disperse coagulant and promote collisions between particles to form microflocs.
• C. To remove solids by gravity.
• D. To measure chlorine residual.

Answer: (B)

Rapid mix is about distributing coagulant quickly to water particles to overcome charge repulsion and encourage collisions that form microflocs. When coagulants are added, fast, high-shear mixing spreads them throughout the water so they contact many particles at once. This rapid interaction neutralizes charges and helps particles stick together, creating tiny aggregates called microflocs in a very short time. Those microflocs then grow into larger flocs during the downstream flocculation stage with gentler mixing. If mixing is too slow or uneven, the coagulant won’t disperse effectively, collisions will be infrequent, and coagulation will be poor. The other options refer to steps that happen later (gravity-based sedimentation) or to unrelated testing (measuring chlorine residual).